This invention relates to a method and composition useful in inhibiting catalyzed oxidation of carbon-carbon composites.
Many aircraft utilize a stack of carbon composite discs in brakes that can absorb large amounts of kinetic energy required to stop the aircraft during landing or in the event of a rejected take-off. During some of the stops, the carbon is heated to sufficiently high temperatures that surfaces exposed to air will oxidize. Carbon composites with thermal and mechanical properties required for specific brake designs have been prepared. However, these composites have had residual open porosities (typically 5% to 10%) which permit internal oxidation. The internal oxidation weakens material in and around the brake rotor lugs or stator slots. These areas transmit the torque during braking. One simple, low-cost method of minimizing loss of strength and structural integrity is application of phosphoric acid to non-wear surfaces of brake discs, followed by baking to 650xc2x0 C. This method inhibits normal oxidation of carbon sufficiently for many applications, including aircraft brakes.
Similarly, carbon-carbon composites have been coated with barriers to prevent oxidation. The barriers include silicon based coatings, such as silicon carbide. The coatings however have cracks and porosity. These cracks and porosity are areas which may be subject to the above oxidation problems.
Some commercial transport brakes have suffered crushing in the lugs or stator slots. The damage has been associated with visible surface oxidation and oxidation enlargement of cracks around fibers or in the carbon-carbon carbon composite coating. The enlargement occurs at depths up to 0.5 inch beneath the exposed surfaces. Potassium or sodium has been identified in the severely oxidized regions. Alkali and alkaline earth elements are well known to catalyze carbon oxidation. Catalyzed oxidation is carbon oxidation that is accelerated by the presence of contaminating materials. These contaminating materials come into contact with the brake from cleaning and de-icing chemicals used on aircraft. Sodium can originate from salt deposits left from seawater or sea spray. These liquids, and other deicers or cleaners containing K or Na, can penetrate the porous carbon discs leaving catalytic deposits within the pores. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude. There is a need to provide protection against such catalyzed oxidation.
McKee points out that phosphates can deactivate catalytic impurities in carbon by converting them to inactive, stable phosphates (D. W. McKee, Chemistry and Physics of Carbon 16, P. I. Walker and P. A. Thrower eds., Marcel Dekker, 1981, p. 30.)
Woodburn and Lynch (U.S. Pat. No. 2,685,539) describe several ways of impregnating pores in carbon or graphite with aluminum phosphate. They specified compositions having a molar ratio of Al2O3:P2O5 between 0.2:1 and 0.8:1. Application methods included brushing, spraying or soaking in solutions, including Al(H2PO4)3 dissolved in aqueous HCl.
U.S. Pat. No. 4,439,491, issued to Wilson, relates to carbon or graphite protected against oxidation by application of a solution comprising mono-ammonium phosphate, zinc orthophosphate, phosphoric acid, boric acid, cupric oxide, and wetting agent in water.
U.S. Pat. No. 4,837,073, issued to McAllister et al., relates to a barrier coating and penetrant providing oxidation protection for carbon-carbon materials. The method involves penetrating a barrier coating for carbon-carbon materials with an oxidation inhibitor.
It is desirable to have a simple, effective method and composition to inhibit catalyzed oxidation of carbon.
This invention relates to a method of inhibiting catalyzed oxidation of carbon-carbon composites having pores comprising the steps of treating a carbon-carbon composite with a liquid composition comprising (a) phosphoric acid, (b) a zinc salt, and (c) an aluminum salt and heating the treated carbon-carbon composite to a temperature sufficient to form deposits within the pores. In certain preferred embodiments the composition is a solution, preferably aqueous-based. This solution may be used in combination with a silicon carbide barrier coating, applied by either painting or by CVD, to allow application at higher temperatures. In one aspect, the deposit comprises a dispersion of aluminum-rich phosphate in zinc-rich phosphate. It is believed that the deposit acts as a scavenger to capture Na or K or other oxidation catalysts. Articles, including friction brakes, and the liquid compositions are also part of this invention. The methods and articles show improved penetration and retention of the catalyzed oxidation inhibitors.